Modeling & simulation of semiconductor manufacturing equipment & processes is an inherently multi-disciplinary effort drawing from such diverse fields as plasma physics/chemistry and heat and mass transfer. Over the past few years, significant strides have been made to address and incorporate several of these phenomena, and enable optimization of chamber geometry and processes through modeling. However, many of these physical phenomena occuring in processing equipment are not even well understood or characterized. Obtaining self-consistent models that combine the influences of different phenomena or different aspects of processes is perhaps the major hurdle between where we are now and taking comprehensive reactor models (or the Virtual Reactor) mainstream. There are several issues that must be resolved before comprehensive reactor models may become commonplace. The computer model must be able to handle stiff chemistry-equations and the resulting numerical convergence problems. In plasma modeling, the sheath must be resolved properly to predict the ion flux and energy at the wafer surface. The extension of fluid models into the transition regime must be incorporated in the commercial codes. Finally ways must be explored to combine the reactor level model with the device/feature level. Currently setting up a model may be very time-consuming due to the lack of smart grid generation tools. The proliferation of modeling to the common engineer cannot happen without streamlining and minimizing the grid-generation effort. Lastly, there is a lack of fundamental or often tool-specific data necessary to accurately simulate these processes. Most of the reaction mechanisms/pathways of relevance to real processes are not well understood. A long-term sustained research effort is required from chip-makers, equipment companies, academia, national laboratories and commercial software developers to overcome these shortcomings.